1. Field of the Invention
This invention relates to flexure suspension assemblies for inertial devices such as gyroscopes and the like and more particularly to a flexure suspension assembly having two degrees of angular rotational freedom and to a method of making the same.
2. Description of the Prior Art
Certain types of gyroscopes and other inertial sensing devices employ a flexure suspension assembly to connect a rotatable spin shaft to a flywheel or inertial member so that the flywheel or inertial member may be brought up to a suitably high speed by rotation of the spin shaft. The spin shaft is fixedly mounted against angular rotation but the flywheel of the gyroscope or other inertial device must be free to "precess" or angularly rotate about two precession axes which are orthogonally related to the spin axis of the spin shaft. Accordingly, the flexure suspension assembly must be rotatable about the spin axis and must have two degrees of angular rotational freedom about the two precession axes. The suspension assembly must also provide adequate axial structural support for the flywheel, since the weight of the flywheel is ultimately borne by the spin shaft. When the spin shaft is rotated to bring the flywheel to full spin speed, the suspension assembly must provide the required radial support. Accordingly, the flexure suspension assembly provides both axial and radial support for rotation of the flywheel. The support provided must prevent translatory movement of the flywheel with respect to the spin shaft along the spin axis and the two orthogonally related precession axes when the gyroscope or other inertial device is subjected to external acceleration forces along these axes. The foregoing structural support must be provided, however, without imposing substantial flexural restraints on the angular rotation of the flywheel about the two precession axes or error torques will be produced during gyroscope operation which will degrade the accuracy of the gyroscope output information.
Flexure suspension assemblies have been produced by a number of different methods which utilize the formation of single axis flexure joints. Single axis flexure joints are essentially thin sections of metal or other suitable material of which the suspension assembly is made which may be bent or flexed about only a single axis. A single axis flexure joint may be formed by drilling or otherwise forming a pair of closely spaced parallel holes in a member and separating the member into two portions which are joined only by the thin section of material between the closely spaced holes. This will produce a flexure joint which may be bent or flexed about a single bending axis which is midway between the holes and is parallel to the two longitudinal axes of the holes. The longitudinal axis of the flexure joint would then be perpendicular to the bending axis and would be located equidistantly between the two holes.
A known method of producing a flexure suspension assembly having single axis flexure joints involves the use of a single cylindrical member in which four single axis flexure joints are distributed about the periphery of the member at 90.degree. intervals by the boring of four pairs of closely spaced and radially separated holes. The longitudinal axes of these flexure joints are substantially parallel to the longitudinal axis of the cylinder and the bending axes are radially disposed. The single cylinder is then separated by known techniques into spin shaft and flywheel mounting members and gimbal members which are interconnected by eight single axis flexure joints all having their longitudinal axes parallel to the longitudinal cylinder axis. In the resulting flexure suspension assembly, the flywheel is supported radially by four of the axially-extending flexure joints loaded in shear. Axial support for the flywheel is provided by all of the eight axial flexure joints loaded in tension. Although the resulting suspension assembly provides good axial support for the flywheel, it has the disadvantage of providing poor radial support. In order to increase the radial support provided by this type of suspension assembly, the single axis flexure joints must be made stronger. However, the strengthening of the flexure joints also increases the stiffness of the joints and increases the flexural restraints imposed by the suspension assembly upon the flywheel for angular rotation about the gyroscope precession axes. Although the error torques resulting from the increased flexural restraints could be minimized by adjusting or tuning the moments of inertia of the gimbal members of the assembly, the tuning is made very difficult by the increased stiffness of the flexure joints.
Other methods of making flexure suspension assemblies involve the use of two separate cylindrical members which are individually separated or machined into the requisite number of flexure joints and other component parts of the flexure suspension assembly prior to joining. After each cylindrical member has been separated into its share of the component parts of the assembly, the machined members are joined together at the flexure joints produced in each member to provide the final flexure suspension assembly. This method also produces a suspension assembly which suffers from the structural disadvantages of the assemblies produced by the single cylinder method. Additionally, the known methods and suspension assemblies utilize a technique for mounting the flexure suspension assembly on the gyroscope spin shaft wherein a connecting shaft is formed or machined as a part of the spin shaft mounting member. The connecting shaft is then blind cemented into a recess in the gyroscope spin shaft. With this arrangement, the distribution of the flywheel inertial load is no longer centrally located at the shaft mounting surface but is instead cantilevered away from the cement joint mounting surface. This may produce a source of gyroscope drift errors which would not exist if the flexure suspension assembly could be centrally mounted on the gyroscope spin shaft with the flywheel inertial load evenly supported at the shaft mounting surface.